Process to prepare a hydrogen and carbon monoxide containing gas

ABSTRACT

A process for the preparation of hydrogen and carbon monoxide containing gas from a gaseous hydrocarbon feedstock by performing the following steps:  
     (a) partial oxidation of part of the feedstock thereby obtaining a first gaseous mixture of hydrogen and carbon monoxide; and,  
     (b) catalytic steam reforming of part of the gaseous feedstock in a Convective Steam Reformer having a tubular reactor provided with one or more tubes containing a reforming catalyst, wherein the exterior of the tubes of the tubular reactor is used to cool the hot gas as obtained in step (a) and wherein the exterior of the tubes is a metal alloy surface having between 0 wt % and 20 wt % iron.

[0001] The invention is directed to a process for the preparation of ahydrogen and carbon monoxide containing product gas mixture from agaseous hydrocarbon feedstock, by subjecting part of the gaseousfeedstock to a partial oxidation step to obtain a first product mixtureand part of the gaseous feedstock to a endothermic reaction in thepresence of steam and/or carbon dioxide performed in a fixed bed tubularreactor to obtain a second product mixture, wherein the first productmixture is reduced in temperature by contacting said gas with theexterior of the tubular reactor.

[0002] Such a process is described in EP-A-168892 of applicant in 1986.According to this publication the endothermic reaction is preferablycarried out in a fixed bed situated in at least one pipe in which atemperature of between 800 and 950° C. is maintained by routing at leastpart of the hot product gas from the partial oxidation along thepipe(s). According to this publication the combined partial oxidationand endothermic production of synthesis gas result in a better yield ofsynthesis gas, an increased H₂/CO ratio, a lower usage of oxygen per M³of syngas product obtained and a lower capital cost of the plant for theproduction of CO and H₂-containing gas mixtures. An example of theprocess as described in EP-A-168892 is described in EP-A-326662.

[0003] EP-A-171786 discloses a similar process as EP-A-168892. Thedifference is that the product gas having the elevated temperature isnot prepared by means of partial oxidation of natural gas but in aconventional reformer furnace wherein burners provide the required heat.This first process gas is then cooled in a so-called Enhanced HeatTransfer Reformer (EHTR) by routing this gas along the exterior oftubular pipes of the EHTR. EHTR reactors and the like are generallyreferred to as Convective Steam Reformer (CSR). These pipes contain afixed bed of catalyst for performing an endothermic reforming reactionusing a second part of the natural gas feed. The mixture of carbonmonoxide and hydrogen as obtained within the tubes could be consideredto be the second product gas according this invention. The product gasas obtained in the conventional reformer contains approximately 33%steam.

[0004] U.S. Pat. No. 6,224,789 discloses a similar process as describedabove except that the product gas having the elevated temperature isprepared from natural gas in a so-called Autothermal Reformer (ATR) inthe presence of a Ni-containing catalyst and steam. The hot product gasis then contacted with the exterior of the reactor tubes of an EHTR likereactor.

[0005] The tubes of a Convective Steam Reformer are typically made frommetal alloys comprising substantially of iron. Iron containing alloysare preferred because of their mechanical strength in combination withtheir relative low cost. Furthermore usage of these alloys makes itpossible to manufacture the complicated tube structures of such anapparatus. A disadvantage of the above apparatus is that in use cokewill form on the exterior surface of the tubes because part of thecarbon monoxide reacts to carbon and carbon dioxide. Furthermore part ofthe surface will erode resulting eventually in an unacceptable lowmechanical integrity of the tubes. These effects are especiallysignificant when the amount of steam in the hot gas is below 50 vol %.Such a hot CO and H₂ containing gas is for example obtained whenperforming a partial oxidation of natural gas, refinery gas, methane andthe like in the absence of added steam as described in WO-A-9639354.There is thus a need for an improved process if one intends to operate apartial oxidation and a reforming process in combination, as for exampledescribed in EP-A-168892 or in EP-A-326662.

[0006] The object of the present invention is to provide a processhaving the advantages of the process of EP-A-168892 or EP-A-326662wherein less or no coke formation and/or erosion on the exterior of thereactor tubes occurs.

[0007] This object is achieved when the following process is used. Aprocess for the preparation of hydrogen and carbon monoxide containinggas from a gaseous hydrocarbon feedstock by performing the followingsteps:

[0008] (a) partial oxidation of part of the feedstock thereby obtaininga first gaseous mixture of hydrogen and carbon monoxide and

[0009] (b) catalytic steam reforming of part of the gaseous feedstock ina Convective Steam Reformer comprising a tubular reactor provided withone or more tubes containing a reforming catalyst, wherein the exteriorof the tubes of the tubular reactor is used to cool the hot gas asobtained in step (a) and wherein the exterior of the tubes is a metalalloy surface comprising between 0 and 20 wt % iron.

[0010] Applicants found that less erosion and coke formation will occuron the exterior of the reactor tubes of the CSR if a low iron metalalloy surface is applied. It becomes possible to combine the partialoxidation of natural gas as performed in the absence of (a substantialamount of) steam as moderator gas, i.e. generating a hot gas having asteam content of below 50 vol % and more preferred below 15 vol % with aCSR process. The combination of a partial oxidation and a CSR process isfurthermore advantageous because the hot gas generated by the partialoxidation has a higher temperature than the hot gas generated by theconventional reformer. This will enable one to process relatively morenatural gas through the CSR and/or making it possible to operate at ahigher conversion of said gas because of the higher possible exittemperature of the catalytic steam reforming section of the CSRapparatus. Preferably the weight ratio of natural gas processed in step(a) and in step (b) is between 0.5 and 3. Another advantage is that thehydrogen to carbon monoxide ratio can be lower relative to the processas disclosed in U.S. Pat. No. 4,919,844, which is advantageous when sucha gas is used as feedstock for a Fischer-Tropsch synthesis process,methanol synthesis process or DME synthesis process. Preferred H₂/COmolar ratio's of the total synthesis gas product as obtained by theabove combined process is between 1.9 and 2.3.

[0011] In step (a) the partial oxidation may be performed according towell known principles as for example described for the ShellGasification Process in the Oil and Gas Journal, Sep. 6, 1971, pp 85-90.Publications describing examples of partial oxidation processes areEP-A-291111, WO-A-9722547, WO-A-9639354 and WO-A-9603345. In suchprocesses the feed is contacted with an oxygen containing gas, such asair or pure oxygen or a mixture thereof, under partial oxidationconditions. Contacting is preferably performed in a burner placed in areactor vessel. Preferably the partial oxidation is performed in theabsence of significant amounts of added steam, and preferably in theabsence of added steam, as moderator gas. The gaseous feed is forexample natural gas, refinery gas, associated gas or (coal bed) methaneand the like.

[0012] The product gas of step (a) preferably has a temperature ofbetween 1100 and 1500° C. and a H₂/CO molar ratio of between 1.5 and2.6, preferably between 1.6 and 2.2.

[0013] Step (b) may be performed by well known steam reformingprocesses, wherein steam and the gaseous hydrocarbon feed are contactedwith a suitable reforming catalyst in a CSR reactor. Suitable processesof are exemplified in the earlier referred to US-B1-6224789 andEP-A-171786. The steam to carbon (as hydrocarbon and CO) molar ratio ispreferably between 0 and 2.5 and more preferably between 0.5 and 1.Preferably the feed also comprises an amount of CO₂, wherein preferablythe CO₂ over carbon (as hydrocarbon and CO) molar ratio is between 0.5and 2. The product gas of step (b) preferably has a temperature ofbetween 600 and 1000° C. and a H₂/CO molar ratio of between 0.5 and 2.5.

[0014] The gaseous feedstock to both step (a) and (b) may also compriserecycle fractions comprising hydrocarbons and carbon dioxide as may beobtained in earlier referred to downstream processes, such as theFischer-Tropsch process, which use the CO/H₂ containing gas asfeedstock.

[0015] The invention is also related to CSR reactor vessel comprisingreactor tubes having a metal alloy surface as exterior and a metal alloysupport as the interior.

[0016] The temperature of the hydrogen and carbon monoxide containinggas is preferably reduced in step (b) from a temperature of between 1000and 1500° C. to a temperature between 300 and 750° C. The temperature ofthe alloy surface in step (b) is preferably below 1100° C.

[0017] The mixture of carbon monoxide as obtained in step (b) may bedirectly combined with the product gas as obtained in step (a). This maybe achieved within the CSR reactor as exemplified in U.S. Pat. No.4,919,844. Alternatively the product gas as obtained in step (b) may befed to step (a) such that the combined mixture is used to cool thereactor tubes of the CSR reactor in step (b).

[0018] The present invention is thus directed to a process to reduce thetemperature of a hydrogen and carbon monoxide containing gas as preparedby a partial oxidation process by contacting the gas with a metal alloysurface having a lower temperature than the temperature of the gas,wherein the metal alloy surface comprises between 0 and 20 wt % andpreferably between 0 and 7 wt % iron. The alloy surface preferably alsocontains between 0 and 5 wt % aluminium, preferably between 0 and 5 wt %silicon, preferably between 20 and 50 wt % chromium and preferably atleast 35 wt % nickel. Preferably the nickel content balances the totalto 100%. The metal alloy surface is preferably supported with a metalalloy support layer having better mechanical properties than saidsurface layer.

[0019] It has been found beneficial to have at least some aluminiumand/or silicon in the metal alloy surface when the concentration ofsteam in the hot gas is lower than 50 vol %, preferably lower than 30vol % and more preferably lower than 15 vol %. Preferably between 1-5 wt% aluminium and between 1-5 wt % silicon is present in said alloy layerunder such low steam content conditions. The resulting aluminium oxideand silicon oxide layers will provide an improved protection againstcoke formation and erosion when the conditions become more reducing atsuch low steam concentrations. More preferably next to aluminium andsilicon a small amount of titanium and/or REM (reactive elements) areadded to the metal alloy. Examples of REM are Y₂O₃, La₂O₃, CeO₂, ZrO₂and HfO₂. The total content of these added compounds is between 0 and 2wt %.

[0020] The metal alloy support layer may be any metal alloy having therequired mechanical strength for a particular application. Typicallythese metal alloys will contain more iron than the surface layer,suitably more than 7 wt % and even up to 98 wt %. Other suitable metals,which can be present in this metal alloy, are chromium, nickel andmolybdenum. Examples of suitable metal allow support layers are carbonsteels, austenitic stainless steels, for example the AISI 300 series(examples 304, 310, 316) with a typical Cr content of between 18-25% andNi content of between 8-22%, cast materials, like for example HK-40,HP-40 and HP-modified, nickel based alloys, for example Inconel 600,Inconel 601, Inconel 690 and Incoloy 800 and ferritic stainless steels,which are Fe based alloys having a low nickel content, e.g. less than 2wt % and a Cr content of above 12 wt %.

[0021] The two layers of metal alloys may be prepared by methods knownto one skilled in the art. Preferably the metal alloy composite is madeby means of a building-up welding method resulting in a weld-mountedmulti-layered metal surface. This method is preferred because it enablesone to make difficult tubular structures, as used in a CSR reactor,having the metal alloy surface according to the present invention. Sucha method is characterized in that the desired metal alloy for use as thesurface layer is first atomized by gas atomization to form a powder ofsaid alloy. Preferably the iron content of said powder is substantiallyzero. A layer of the metal alloy is subsequently applied on the supportmetal alloy by built-up welding by plasma powder welding of said powder.After machining the weld metal a flat metal alloy surface is obtained.Thickness of the surface metal alloy may range from 1 to 5 mm andpreferably 1 to 3 mm. It has been found that the iron content in themetal alloy layer may contain iron in a situation wherein the staringpowder did not contain iron. This is due to migration of iron from thesupport layer to the surface layer during the welding step. Care shouldbe taken to limit the migration of iron to the surface layer such thatthe end iron content in the surface layer will be below 20 wt % andpreferably below 7 wt %. The iron migration effect can be limited byusing a low iron-content support layer, increasing the layer thicknessand/or by applying the layer in more than one step. A preferred methodto perform such a building-up welding method is described inEP-A-1043084, which publication is hereby incorporated by reference.This publication describes a method to obtain coke resistant furnacereactor tubes for a steam cracker process, which is aimed at preparinglower olefins, e.g. ethylene and propylene.

1. A process for the preparation of hydrogen and carbon monoxide containing gas from a gaseous hydrocarbon feedstock by performing the following steps: (a) partially oxidating part of the feedstock thereby obtaining a first gaseous mixture of hydrogen and carbon monoxide; and, (b) steam reforming part of the gaseous feedstock in a convective steam reformer comprising a tubular reactor provided with one or more tubes containing a reforming catalyst, wherein the exterior of the tubes of the tubular reactor is used to cool the hot gas as obtained in step (a) and wherein the exterior of the tubes is a metal alloy surface comprising between 0 wt % and 20 wt % iron and between 1 wt % and 5 wt % silicon.
 2. The process of claim 1, wherein the metal alloy surface further comprises between 0 wt % and 5 wt % aluminium, between 20 wt % and 50 wt % chromium and at least 35 wt % nickel and wherein the metal alloy surface is supported with a metal alloy support layer having better mechanical properties than said surface layer.
 3. The process of claim 2, wherein the metal alloy comprises more than 30 wt % chromium.
 4. The process of claims 2, wherein the metal alloy surface comprises between 1 wt % and 5 wt % aluminium
 5. The process of claim 4, wherein the metal alloy surface comprises between 0 wt % and 2 wt % titanium and/or reactive elements.
 6. The process of claims 2, wherein the metal alloy support layer comprises between 7 wt % and 98 wt % iron.
 7. The process of claims 2, wherein the metal alloy surface layer is applied to the metal alloy support layer by means of a building-up welding method.
 8. The process of claims 1, wherein the temperature of the hydrogen containing gas of step (a) is reduced from a temperature of between 1000° C. and 1500° C. to a temperature between 300° C. and 750° C. in step (b).
 9. The process of claims 1, wherein the hot gas of step (a) has a hydrogen to CO molar ratio of between 1.5 and 2.5.
 10. The process of claims 1, wherein the hot gas used in step (b) comprises less than 15 vol % steam.
 11. The process of claims 1, wherein the gaseous feed in step (b) comprises a hydrocarbon gas, steam and carbon dioxide and wherein the steam to carbon molar ratio is between 0.5 and 1 and the CO₂ over carbon molar ratio is between 0.5 and 2 